DESCRIPTION

These system calls create and operate on a timer that delivers timer
expiration notifications via a file descriptor. They provide an
alternative to the use of setitimer(2) or timer_create(2), with the
advantage that the file descriptor may be monitored by select(2),
poll(2), and epoll(7).
The use of these three system calls is analogous to the use of
timer_create(2), timer_settime(2), and timer_gettime(2). (There is no
analog of timer_getoverrun(2), since that functionality is provided by
read(2), as described below.)
timerfd_create()timerfd_create() creates a new timer object, and returns a file
descriptor that refers to that timer. The clockid argument specifies
the clock that is used to mark the progress of the timer, and must be
either CLOCK_REALTIME or CLOCK_MONOTONIC. CLOCK_REALTIME is a settable
system-wide clock. CLOCK_MONOTONIC is a nonsettable clock that is not
affected by discontinuous changes in the system clock (e.g., manual
changes to system time). The current value of each of these clocks can
be retrieved using clock_gettime(2).
Starting with Linux 2.6.27, the following values may be bitwise ORed in
flags to change the behavior of timerfd_create():
TFD_NONBLOCK Set the O_NONBLOCK file status flag on the new open file
description. Using this flag saves extra calls to
fcntl(2) to achieve the same result.
TFD_CLOEXEC Set the close-on-exec (FD_CLOEXEC) flag on the new file
descriptor. See the description of the O_CLOEXEC flag in
open(2) for reasons why this may be useful.
In Linux versions up to and including 2.6.26, flags must be specified
as zero.
timerfd_settime()timerfd_settime() arms (starts) or disarms (stops) the timer referred
to by the file descriptor fd.
The new_value argument specifies the initial expiration and interval
for the timer. The itimer structure used for this argument contains
two fields, each of which is in turn a structure of type timespec:
struct timespec {
time_t tv_sec; /* Seconds */
long tv_nsec; /* Nanoseconds */
};
struct itimerspec {
struct timespec it_interval; /* Interval for periodic timer */
struct timespec it_value; /* Initial expiration */
};
new_value.it_value specifies the initial expiration of the timer, in
seconds and nanoseconds. Setting either field of new_value.it_value to
a nonzero value arms the timer. Setting both fields of
new_value.it_value to zero disarms the timer.
Setting one or both fields of new_value.it_interval to nonzero values
specifies the period, in seconds and nanoseconds, for repeated timer
expirations after the initial expiration. If both fields of
new_value.it_interval are zero, the timer expires just once, at the
time specified by new_value.it_value.
The flags argument is either 0, to start a relative timer
(new_value.it_value specifies a time relative to the current value of
the clock specified by clockid), or TFD_TIMER_ABSTIME, to start an
absolute timer (new_value.it_value specifies an absolute time for the
clock specified by clockid; that is, the timer will expire when the
value of that clock reaches the value specified in new_value.it_value).
If the old_value argument is not NULL, then the itimerspec structure
that it points to is used to return the setting of the timer that was
current at the time of the call; see the description of
timerfd_gettime() following.
timerfd_gettime()timerfd_gettime() returns, in curr_value, an itimerspec structure that
contains the current setting of the timer referred to by the file
descriptor fd.
The it_value field returns the amount of time until the timer will next
expire. If both fields of this structure are zero, then the timer is
currently disarmed. This field always contains a relative value,
regardless of whether the TFD_TIMER_ABSTIME flag was specified when
setting the timer.
The it_interval field returns the interval of the timer. If both
fields of this structure are zero, then the timer is set to expire just
once, at the time specified by curr_value.it_value.
Operatingonatimerfiledescriptor
The file descriptor returned by timerfd_create() supports the following
operations:
read(2)
If the timer has already expired one or more times since its
settings were last modified using timerfd_settime(), or since
the last successful read(2), then the buffer given to read(2)
returns an unsigned 8-byte integer (uint64_t) containing the
number of expirations that have occurred. (The returned value
is in host byte order, i.e., the native byte order for integers
on the host machine.)
If no timer expirations have occurred at the time of the
read(2), then the call either blocks until the next timer
expiration, or fails with the error EAGAIN if the file
descriptor has been made nonblocking (via the use of the
fcntl(2) F_SETFL operation to set the O_NONBLOCK flag).
A read(2) will fail with the error EINVAL if the size of the
supplied buffer is less than 8 bytes.
poll(2), select(2) (and similar)
The file descriptor is readable (the select(2) readfds argument;
the poll(2) POLLIN flag) if one or more timer expirations have
occurred.
The file descriptor also supports the other file-descriptor
multiplexing APIs: pselect(2), ppoll(2), and epoll(7).
close(2)
When the file descriptor is no longer required it should be
closed. When all file descriptors associated with the same
timer object have been closed, the timer is disarmed and its
resources are freed by the kernel.
fork(2)semantics
After a fork(2), the child inherits a copy of the file descriptor
created by timerfd_create(). The file descriptor refers to the same
underlying timer object as the corresponding file descriptor in the
parent, and read(2)s in the child will return information about
expirations of the timer.
execve(2)semantics
A file descriptor created by timerfd_create() is preserved across
execve(2), and continues to generate timer expirations if the timer was
armed.

RETURNVALUE

On success, timerfd_create() returns a new file descriptor. On error,
-1 is returned and errno is set to indicate the error.
timerfd_settime() and timerfd_gettime() return 0 on success; on error
they return -1, and set errno to indicate the error.

ERRORS

timerfd_create() can fail with the following errors:
EINVAL The clockid argument is neither CLOCK_MONOTONIC nor
CLOCK_REALTIME;
EINVALflags is invalid; or, in Linux 2.6.26 or earlier, flags is
nonzero.
EMFILE The per-process limit of open file descriptors has been reached.
ENFILE The system-wide limit on the total number of open files has been
reached.
ENODEV Could not mount (internal) anonymous inode device.
ENOMEM There was insufficient kernel memory to create the timer.
timerfd_settime() and timerfd_gettime() can fail with the following
errors:
EBADFfd is not a valid file descriptor.
EFAULTnew_value, old_value, or curr_value is not valid a pointer.
EINVALfd is not a valid timerfd file descriptor.
timerfd_settime() can also fail with the following errors:
EINVALnew_value is not properly initialized (one of the tv_nsec falls
outside the range zero to 999,999,999).
EINVALflags is invalid.

VERSIONS

These system calls are available on Linux since kernel 2.6.25. Library
support is provided by glibc since version 2.8.